Speed regulator for asynchronous motors



April 16; 1957 LANNEFORS SPEED REGULATOR FOR ASYNCHRONOUS MOTORS Filed Feb. 20', 1951.

Cl d (7 2 Len narri f United States Patent "cc SPEED REGULATOR FOR ASYNCHRONOUS MOTORS Lennart Lannefors, Ludvika, Sweden, assignor to Allmanna Svenska Elektriska Aktiebolaget, Vasteras, Sweden, a Swedish corporation Application February 20, 1951, Serial No. 211,931 Claims priority, application Sweden February 22, 1950 7 Claims. (Cl. 318229) It has been known before that it is necessary for controlling the speed of an asynchronous motor to vary either the frequency of the motor voltage, the number of poles of the motor, or the slip. The possibility of changing the frequency either requires means for changing the line frequency or necessitates an expensive auxiliary equipment. The possibility of changing the number of poles requires a specially wound machine which does not permit continuous control. None of these methods is therefore applied to speed control of normal asynchronous motors, but the most common way is to cause a variation of the slip.

The most frequently used method for the last mentioned purpose is to connect an adjustable resistor to slip rings on the motor. This method, however, involves the drawback that the speed of the motor will be very dependent on the torque, and this dependence is most pronounced at low speeds.

Besides a secondary connected resistor it has also been proposed to connect adjustable resistors to the primary side of the motor in order to adjust the voltage supplied to the motor. This arrangement, however, only permits adjustment within a very limited range of speed, and it has the same drawbacks as mentioned above regarding the dependence of the speed on the torque.

By means of the present invention, however, said drawback is avoided, and it will be possible to obtain continuous speed control within a wide range. The invention is based on the observation that transductors connected to the primary side of the motor will permit a convenient way of controlling the primary voltage of the motor. By this arrangement the active losses of the control equipment will be very small owing to the transductor being an inductive impedance. On the contrary, the losses will be most appreciable when resistors are used in a corresponding way, in order to adjust the slip.

The characterising feature of the invention is that the transductor is excited by ampereturns obtained from the difference between two currents, one of which is derived from the primary or secondary voltage of the motor and the other from the primary or secondary current of the motor.

With an arrangement according to the invention it is possible to control astock motor automatically so that difierent speeds may be set independent of the torque of the load. The motor need not be altered and not be altered and not even equipped with a tachometer-generator or frequency dependent members, and its control properties will not vary with the character of the load.

The invention will be further described with reference to the accompanying drawing in which Figure 1 shows a diagram of the torque of the motor as a function of the slip, and

Figure 2 diagrammatically shows one form of the invention, and

Figure 3 diagrammatically shows another form of the invention.

In Figure 1, the torque is designated by M and the slip 2,789,264 ate nted Apr. 16, 1957 by s, and as usual s=0 represents the synchronous speed of the motor and s=1 a locked rotor. In the diagram, a and b are curves for different resistances in the rotor circuit, depending on the magnitude of the resistance connected to the slip rings. The resistance according to curve a is then larger than the resistance according to curve b. m and as are curves for the same rotor resistance as a but for a further decreased voltage on the motor. The dashed curves 0 and d are torque curves at diiferent pre-set speeds for the system constituted by motor and control equipment.

In the form according to Figure 2, one terminal of the alternating current windings of a three phase transductor 1 is connected to an alternating current line 2. The other terminal of said windings is connected to the stator of the asynchronous motor 3, for each phase in series with the primary winding of a current transformer 4. A three phase adjustable resistor 5 is connected to the slip rings of the motor. The secondary windings of the current transformers 4 are three phase connected to a rectifier bridge 6 in two-way six-pulse connection, from which direct current is supplied to an exciting Winding 7 on the transductor 1 in parallel with an adjustable resistor 8, by means of which the exciting current may be adjusted. On the primary side of the motor a rectifier bridge 9 is connected between two phases. From the direct current side of the last mentioned bridge a second exciting winding 11 of the transductor 1 is fed in series with an adjustable resistor 10. This winding 11 is so arranged that its magnetic field counteracts that of the winding 7.

In Figure 3, a somewhat modified form is shown. The motor 3 is connected to an alternating current line 2 in series with the alternating current windings of a three phase transductor 1. The rotor side of the motor is connected to a two-way six-pulse rectifier 6, from which direct current is supplied, in series with an adjustable resistor 12 and in parallel with a further adjustable resistor 8, to an exciting winding 7 of the trans-- ductor 1. A second counter-acting exciting winding 11 is connected in the same Way as described in connection with Figure 2 via a resistor 10 and a rectifier 9 to twophases of the primary voltage of the motor.

It is possible to omit one of the resistors 7 and 10 shown in the Figures 2 and 3 without the properties of' the arrangement being changed. It is equally possible to connect current transformers 4 to fewer than three phases or to change the rectifier bridges 6 and 9 to a. lower or higher number of phases.

For understanding the mode of operation of the arrangement it must be considered that the impedance of 'the asynchronous motor is entirely determined by the slip s for a certain secondary resistance. It is further necessary to presume that each of the ampereturns de-- termined by the current through the counteracting windings 7 and 11 on the transductor is as large as possible, in order that the transductor shall operate with the high-- est possible sensibility.

The mode of operation of the arrangement according to Figure 2 will be described in connection with Figure lv for an assumed increase in torque according to this diagram AM. The motor is then assumed to operate with constant rotor resistance determined by the curve (1 at the voltage on the primary side of the motor corresponding to this curve. To begin with, this voltage is supposed to be lcept constant. When the moment is increased with the amount AM, s is changed according to curve a in such a direction that the speed of the motor will drop. When .9 is increased, however, the impedance of the motor will be decreased, which is clear from the expression for the impedance containing a term which is inversely proportional to s. As a consequence, the motor current will increase with a certain value. For this reason also the current supplied from the current transformers 4 through the rectifier 6 will increase, and thus also the current through the winding 7 on the transductor. This increase of the exciting current, however, causes the voltage drop across the transductor to decrease, so that a higher voltage remains for the motor. The voltage across the rectifier 9 will then also be increased, resulting in an increased current traversing the other exciting winding 11 of the trans ductor. The transductor will, however, be in balance when its total number of ampereturns is 0, and will then operate on that part of its curve which has the greatest steepness. It is now possible to drop the presumption that the voltage is kept constant according to curve a,. In reality, the control is accomplished quite continuously, so that a certain rise of the motor current is corresponded by an adapted rise of the motor voltage. Through the influence of the transductor the voltage has thus been increased, so that it corresponds to the curve (1,. At that voltage the rate between voltage and current of the motor is still constant, i. e. the impedance is the same as before the change of the torque, and consequently also the speed is constant. A curve is obtained, from which it is evident that the speed becomes independent of an appreciable change in the torque. By adjusting anyone of the resistors 5, 8 or 10, another speed may be set, for instance corresponding to curve 9 in Figure 1.

The device according to Figure 3 has a mode of operation principally analogous to that just described. The comparison ampereturns of the exciting windings of the transductor are in this case derived from the primary voltage of the motor for the winding 11 and from the secondary current of the motor for the Winding 7. This form is suitable for small motors, whereas the form according to Figure 2 also may be used at motors with higher power.

In the forms shown in the figures the difference between currents forming the exciting ampereturns of the transductor has been accomplished by means of two counter-acting windings on the transductor. It is, how ever, within the scope of the invention that this current difierence also may be accomplished outside the transductor and then supplied to a single exciting winding on it.

I claim as my invention:

1. A speed regulating system for an asynchronous motor, comprising a source of alternating current, a transductor, means connecting said motor to said current source in at least two phases in series with the said transductor, direct current magnetizing'means on said transductor, means for deriving a current substantially proportional to the voltage of said motor, means for deriving a current substantially proportional to the current of said motor, means for supplying the ditference of said derived currents to said direct current magnetizing means of the transductor, and means for adjustably varying at least one of said derived currents.

2. A speed regulating system according to claim 1, in which the said current proportional to the voltage of the motor is derived from the primary voltage of said motor and the said current proportional to the current of the motor is derived from the primary current of said motor.

3. A speed regulating system according to claim 1, in which the said current proportional to the voltage of the motor is derived from the primary voltage of said motor and the said current proportional to the current of the motor is derived from the secondary current of said motor.

4. An alternating-current motor speed control system, comprising a multi-phase wound-rotor motor having a primary circuit and a secondary resistance circuit, voltage control means connected in series in a plurality of the phases of said primary circuit for impressing variable energizing voltage on said motor primary circuit, first variable-voltage supply means connected with said primary circuit and responsive to a load-dependent electric condition of said primary circuit, second variablevoltage supply means connected with said secondary circuit and responsive to a load-dependent electric condition of said secondary circuit, said first and said second voltage supply means being both connected with said control means to jointly control the speed of said motor.

5. An alternating-current motor speed control system, comprising a multi-phase wound-rotor motor having a primary circuit and a secondary resistance circuit, a controllable impedance device series connected in a plurality of the phases of said primary circuit for controlling the speed of said motor and having voltage-responsive impedance control means, condition-responsive circuit means of variable output voltage connected to said control means for componently controlling the impedance of said device, said circuit means being connected with said primary circuit and responsive to a load-dependent electric condition of said primary circuit, and condition-responsive voltage supply means connected with said secondary circuit and attached to said control means for componently controlling said impedance in dependence upon an electric condition of said secondary circuit.

6. An alternating-current motor speed control system, comprising a multi-phase wound-rotor motor having a primary circuit and a secondary resistance circuit, saturable reactance means having a main winding seriesconnected in a plurality of the phases of said primary circuit for controlling the speed of said motor and having control Winding means for controlling the reactance of said main winding, first condition-responsive circuit means connected between said primary circuit and said control winding means and having a variable voltage responsive to a condition of said primary circuit controlled by said reactance, and second condition-responsive circuit means connected between said secondary circuit and said control winding means and having a variable voltage responsive to a condition of said secondary circuit dependent upon said reactance, said two voltages being poled relative to said control winding means in the sense needed for stable speed-torque characteristic of said motor.

7. An alternating-current motor speed control system, comprising a multi-phase wound-rotor motor having a primary circuit and a secondary resistance circuit, saturable reactance means having a main winding seriesconnected in a plurality of the phases of said primary circuit and having control winding means for controlling the reactance of said main winding to control the speed of said motor, voltage-responsive circuit means connected between said primary circuit and said control winding means and having a variable voltage responsive to a voltage of said primary circuit controlled by said reactance, and current-responsive circuit mean connected between said secondary circuit and said control winding means and having a variable voltage responsive to the current in said secondary circuit, said two voltages being poled relative to said control winding means in the sense needed for stable speed-torque characteristic of said motor.

References Cited in the file of this patent UNITED STATES PATENTS 2,433,153 Pell et al. Dec. 23, 1947 2,558,086 Herchenroeder June 26, 1951 2,558,094 King June 26, 1951 2,610,315 McKendry et al. Sept. 9, 1952 

